For backbone cabling and also in connection with Fast-Ethernet and Gigabit-Ethernet, cabling based on optical cables is becoming increasingly more interesting. The signal transmission in optical fibers, also designated as optical waveguides (LWL), is realized via light pulses unidirectionally, i.e., only in one direction, which is why typically at least two optical fibers are used per cable. The light pulses are usually coupled into the fiber by means of a laser diode or a light-emitting diode.
The advantages of optical data transmission consist in the high achievable transmission rates and the large range, as well as in the insensitivity relative to electromagnetic radiation, the protection against eavesdropping, and the resistance to heat and weather effects. Because of the thin inner conductor, optical cables also can have a flexible layout.
In addition to pure glass fibers, optical fibers made from polymers have found a large distribution due to their lower costs. Optical fibers made from polymers are designated as POF fibers (Plastic Optical Fibers) and are pure plastic fibers, which are composed of a transparent core and cladding, wherein the cladding has a lower index of refraction than the core material. Polymers, such as polymethyl methacrylate or polycarbonate, are used as the core material. In addition to the pure polymer fibers, the polymer optical fibers also include hybrid fibers, which consist of a combination of glass fiber and plastic cladding, for example, HCS fibers (Hard Polymer Cladded Silica).
The range that can be directly bridged with optical waveguides is limited by various dispersion and scattering effects. It is dependent on the bandwidth to be transferred and is typically specified as a bandwidth-length product. Relative to glass fibers, polymer fibers, such as POF or HCS have significantly higher attenuation values. POF or HCS fiber systems are operated at path lengths of 50 or 100 in and data rates of 100 Mbps or 125 MBd, which is already at the limits of their technical possibilities. To guarantee reliable data transmission, a diagnosis of the optical fibers of the transmission path is currently a routine procedure.
A method for the diagnosis of optical light waveguide paths, especially for optical diagnosis with an Interbus system, is known, for example, from DE 42 17 899 C2. The method described there is used for optimizing the system of LWL transmission paths during commissioning, and changes the optical transmission power of a transceiver until the optical signal received on the opposite side corresponds to the system requirements.
From EP 1 227 604 A2 a method is known in which an optical transmission path's actual current level reserve is determined relative to the sensitivity limit, that is, one between the current transmission power of the transmission and the current sensitivity limit of the receiver.
The optical diagnosis according to the state of the art has the goal of diagnosing the attenuation of optical connection paths in order to deduce reliable transmission. This is also sufficient for low bit rate systems. For data rates above 100 Mbps, however, in polymer (POF) and HCS fibers, the limiting factor for error-free transmission is no longer the cable attenuation, but instead the bandwidth of the cable. This, however, cannot be automatically tested by the methods known today. Another disadvantage of known systems lies in that the fiber type that is used, such as POF or HCS, which cannot be operated on the same interface, cannot be determined automatically. This also has a disadvantageous effect on the attenuation diagnosis, because different fiber types also have a different attenuation response and thus the diagnostic data must be evaluated differently. Typically, today the fiber type must be specified by hand in the diagnostic software.